Arrangement for driving the drum of a washing machine

ABSTRACT

Arrangement including a self-commutating electric motor for driving the drum of a washing machine. The electric heating element of the washing machine is connected in the supply circuit in series with the motor. This supply circuit further contains a first rectifier which is connected in series with the motor and becomes conducting at the passages through zero of the supply voltage. Speed control of the motor is effected by means of a second rectifier which is of a bidirectional type and which shunts the series combination of the motor and the first rectifier and is set to the conductive condition by means of control pulses. The power dissipated by the heating element may also be regulated by means of this second rectifier.

United States Patent [191 Ebbinge et a1.

Oct. 9, 1973 ARRANGEMENT FOR DRIVING THE DRUM OF A WASHING MACHINE [75]Inventors: Willem Ebbinge; Dirk Cornelis De Ruiter, both of Emmasingel,Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: June 29, 1972 [21] Appl. No.: 267,363

[30] Foreign Application Priority Data July 3, 1971 Netherlands 7109226[52] US. Cl. 307/141, 318/345 [51] Int. Cl. H0lh 7/00 [58] Field ofSearch 307/141; 318/345 [56] References Cited UNITED STATES PATENTS3,638,090 1/1972 Ebbinge et a1 318/345 Primary ExaminerRobert K.Schaefer Assistant ExaminerM. Ginsburg Attorney-Frank R. Trifari [57]ABSTRACT Arrangement including a self-commutating electric motor fordriving the drum of a washing machine. The electric heating element ofthe washing machine is connected in the supply circuit in series withthe motor. This supply circuit further contains a first rectifier whichis connected in series with the motor and becomes conducting at thepassages through zero of the supply voltage. Speed control of the motoris effected by means of a second rectifier which is of a bidirectionaltype and which shunts the series combination of the motor and the firstrectifier and is set to the conductive condition by means of controlpulses. The power dissipated by the heating element may also beregulated by means of this second rectifier.

10 Claims, 5 Drawing Figures ICD PAIENIEUUBT 91% 3,764,822

SHEET 10F 3 PAItNIEU 15 SHEET 3UF .3

ARRANGEMENT FOR DRIVING THE DRUM OF A WASHING MACHINE The inventionrelates to an arrangement including a self-commutating electric motorfor driving the drum of a washing machine, which arrangement is providedwith at least one electric heating element and has a supply circuit forthe motor, which circuit includes the series combination of the heatingelement and a first rectifier, and a switch which is connected acrossthe series combination of the motor and the first rectifier.

Such an arrangement is described, for example, in U.S. Pat. No.3,638,090. The arrangement described in the said Patent is intended toprovide a simple and cheap drum drive in which the heating element,which acts as a series resistor for the motor, is effectively utilized.Owing to this series resistor the motor acquires a kind of highlyexaggerated series characteristic so that its load can be increased toan extent such that it can regularly run at a very low speed and mayeven be blocked without exceeding the permissible motor current. This isof particular importance in driving the drum of an automatic washingmachine because the drum must be capable of rotating at one or a few lowspeeds during the washing cycle and at one or a few high speeds duringthe spinning cycle. In the arrangement described in the said Patent thisis automatically achieved in that during the spinning cycle the loadimposed on the motor is light with a consequent high speed, whereasduring the washing cycle the motor load is heavy and hence owing to theexaggerated series characteristic the speed is very low. At the sametime it is ensured that during the spinning cycle the power dissipatedin the series resistor (heating element) is relatively low because themotor current is then small, whereas during the washing cycle this poweris large because the motor current is large.

The exaggerated series characteristic of the motor further results inthat the transition from washing speed to spinning speed is particularlysmooth. During this transition the motor speed may be increased to thespinning speed before all the wash water has drained away, which, if thedrum speed is appropriately chosen, may involve a very satisfactorydistribution of the washing load.

In this known arrangement, in order to enable the motor speed to beregulated at least at one washing speed, in one embodiment a controlledrectifier is connected in series with the motor. In known manner phasecontrol of the supply voltage is achieved, i.e., the controlledrectifier is rendered conductive for one halfcycle of the supply voltageby the application of a triggering signal to its control electrode andremains conductive for the remainder of this half-cycle. This control ofthe triggering instant enables the motor current and hence the motorspeed to be controlled.

The arrangement described hereinbefore suffers from some limitations anddisadvantages. Firstly the amount of heat generated by the heatingelement is entirely dependent on the operating condition of the motor,because the value of the series resistor is completely determined by thedesired spinning speed. This desired spinning speed also determines theback E.M.F. of the motor at the washing speed and hence the currentflowing through the motor and the series resistor at a given desiredpower. This means that the power dissipated in the series resistor andhence the heat emission during the washing cycle are entirely determinedby the desired spinning and washing speeds, which may prevent thegeneration of heat from reaching a desired high value during the washingcycle. Furthermore the current flowing through the series resistordepends upon the motor control, i.e., upon the triggering instant of thecontrolled rectifier, and hence the generation of heat will also varywith this control.

To enable the wash water to be heated more rapidly at the beginning ofthe washing program an additional heating element may obviously beprovided. However, this additional heating element requires theprovision of an additional power switch which must be switched by theprogram device. A second possibility of increasing the generation ofheat consists in the provision of an additional switch which is capableof directly connecting the heating element to the supply voltage, as isdescribed in the said Patent. This switch may be closed in thestationary condition of the motor so that the heating element deliversits maximum power. However, in this method the power delivered by theheating element is entirely dependent upon the washing rhythm, i.e.,upon the durations of the times during which the drum must rotate and bestationary. These times may be widely different, forexample, for a givenwashing program the drum may be required to rotate for periods of 12seconds each with stationary intervals of 3 seconds, whereas for anotherwashing program the drum may be required to rotate for periods of 3seconds separated by stationary periods of 12 seconds. Obviously theamounts of power dissipated by the heating element will be widelydifferent in these cases. Moreover, the additional power switch will besubject to intense wear.

A second disadvantage of the known arrangement is the risk ofradio-frequency interference owing to the phase control by means of thecontrolled rectifier. This risk of radio-frequency interference is dueto the fact that the rectifier is triggered, and hence the motor currentis switched on, at an instant at which the supply voltage has reached avalue different from zero, possibly even its maximum value, which givesrise to large current variations.

Another disadvantage of the known arrangement is that owing to thecontrol used considerable variations in the line load occur. Thesevariations may be particularly annoying if their repetition frequency islow, for example, 10 Hz, which may even cause the lights connected tothe same line supply to flicker, which obviously is inadmissible.

It is an object of the present invention to provide a drivingarrangement in which, although it is based on the principle of thedescribed known arrangement and has the advantages thereof, thedisadvantages attendant on this known arrangement are largely avoidedand which provides, in addition to simple motor control, simple controlof the power dissipated by the heating element without the need foradditional power switches and for appreciable extension of theprogramming device.

The arrangement according to the invention is characterized in that theswitch consists of a second rectifier which is of a bidirectionalconductivity type and has a control electrode to which a control signalmay be applied which controls the conduction period of the rectifier.

A first advantage of the arrangement according to the invention is thatthe power supplied to, and dissipated by, the heating element isapproximately independent of the operating condition of the motor. Thisis directly due to the provision of the second controlled bidirectionalrectifier and to the fact that the speed of the motor is controlled bymeans of this rectifier. The presence of this controlled rectifierpermits the use of a motor control method different from theconductionangle phase control employed in the known arrangement.

Assuming the motor to be fed during a half-cycle of the supply voltage,this supply may be terminated at any desired instant by triggering thesecond controlled rectifier, for this rectifier shunts the seriescombination of the motor and the first rectifier, so that rendering thissecond rectifier conductive causes the said series combination to beshort-circuited with the result that no current is supplied to themotor. Triggering this sec ond rectifier entails only a limitedvariation of the current flowing through the series resistor. Before thetriggering instant this current is equal to the motor current and afterthe triggering instant the series resistor is directly fed with thesupply voltage. Furthermore, retriggering the second rectifier at anappropriate instant, that is the instant at which the current flowingthrough this rectifier becomes zero, permits of ensuring that the seriesresistor is supplied during the other half-cycle of the supply voltagealso. Thus, in this case the heating element is fed during the entirecycle of the supply voltage and hence dissipates maximum power. When thesecond rectifier is not re-triggered at the aforementioned instant, theheating element is energized during only one half-cycle of the supplyvoltage so that it dissipates only one half of its maximum power.Triggering this second rectifier consequently ensures, in addition tothe motor control, control of the heating effected by the heatingelement.

A second advantage is that the risk of radiofrequency interference canbe appreciably reduced, for the motor current may be switched on at aninstant at which the voltage across the first rectifier is zero, whichinstant is determined by the variation of the supply voltage and thevalue of the back E.M.F. The ensuing transient phenomena and hence theresulting radiofrequency interference will be a minimum so that theanti-interference means required may be reduced to a minimum.

A third advantage is that the variations of the line load areappreciably smaller than in the known arrangement. In the maximumsetting of the heating the heating element is not continually switchedon each time the motor is stopped, but owing to the triggering of thesecond rectifier it is fed during the entire cycle of the supplyvoltage, so that the variations in the line load are negligible.

Finally no additional power switches are required and a simpleprogramming device may be used.

A first embodiment of the arrangement according to the inventionfurthermore enables the direction of the motor to be electronicallyreversed. In this embodiment the first rectifier element connected inseries with the motor is a controlled bidirectional rectifier element.Changing the triggering instants of the first and second rectifierelements permits of reversing the direction of flow of the motorcurrent.

A second embodiment of the arrangement according to the invention ischaracterized in that the first rectifier is a diode. The use of such anuncontrolled rectifier is possible in the arrangement according to theinvention because this rectifier need not be used for the speed control.

Obviously various types of self-commutating motors may be used, forexample, motors with permanentmagnet energization, collector motors withseries, shunt or compound energisation, motors having rotating permanentmagnets, and so on.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which: 7

FIG. 1 is a schematic circuit diagram of an embodiment of the knownarrangement,

FIGS. 2 and 3 are schematic circuit diagrams of two embodiments of thearrangement according to the invention,

FIG. 4 shows the waveforms of the various currents flowing in anarrangement according to the invention, and

' FIG. 5 shows, by way of example, a circuit diagram of an arrangementaccording to the invention including control circuits.

Referring now to FIG. 1, the embodiment shown of the known arrangementincludes a self-commutating electric motor 1, for example, a motorprovided with permanent-magnet energisation. The motor 1 is connected inseries with a heating element 2, which serves as a series resistor forthe motor and the resistance of which is chosen so that the motor has ahighly exaggerated series characteristic. The motor is also connected inseries with a controlled rectifier (thyristor) 3 the control electrode,or gate, of which is connected to a terminal 4. A supply voltage V- isapplied to this series combination of the heating element 2, the motor 1and the thyristor 3 via terminals 6 and 6.

The speed of the motor is controlled by controlling the firing instantof the thyristor 3, i.e., by applying a trigger pulse to the terminal 41at a desired instant. This has the disadvantage that the likelihood ofradiofrequency interference is great, for the thyristor current isswitched on at an instant at which the voltage across this element has apositive value, which may give rise to considerable current variations.

It will further be clear that the current through the heating element 2is entirely determined by the motor current. Current is only supplied tothe heating element during the part of the positive half-cycle of thesupply voltage V- in which the thyristor is conductive, so that thegeneration of heat depends entirely on the motor conditions.

To permit the generation of heat to be increased at a desired instant aswitch 5 is provided which enables the heating element 2 to be directlyconnected to the supply voltage during the stationary periods of themotor. Heat dissipation naturally is entirely dependent on the durationof these stationary periods which may widely differ in the variouswashing programs.

FIG. 2 shows a first embodiment of the arrangement according to theinvention. Similarly to the known arrangement the arrangement shown inFIG. 2 include the series combination of a heating element 2, a motor 1and a controlled rectifier 3", the supply voltage V- being applied tothis series combination via terminals 6 and 6. The rectifier 3" used inthis case is a controlled bidirectional rectifier (triac), however, itmay alternatively be a thyristor. The series combination of the motor 1and the first rectifier 3 in this embodiment is shunted by a secondbidirectional rectifier 7. The triggering signals for both rectifiersare supplied by a control device 8.

The use of the second rectifier 7 provides a speed control for the motorI which is quite different from that used in the known arrangement, aswill be explained with reference to the waveforms of the variouscurrents shown in FIG. 4. At the beginning of a halfcycle of the supplyvoltage V-, for example, at the beginning of the positive half-cycle(instant t a control pulse is applied to the rectifier 3. Hence at thisinstant a motor current I will start flowing, and the current I flowingthrough the heating element is equal to the current I,. At a giveninstant t, during the positive halfcycle of the supply voltage V- thereis applied to the second rectifier 7 a control pulse which renders itconductive, so that the series combination of the motor 1 and therectifier 3 is short-circuited and hence current is no longer suppliedto the motor. Owing to the inductive nature of the motor a motor currentI will remain flowing for a certain period (t,t This motor current flowsthrough the first rectifier 3 and through the second rectifier 7. Thusthe second rectifier 7 also serves as a freewheel diode for the motor.

At the instant t, the heating element 2 is directly connected to thesupply voltage via the second rectifier 7, so that the current I flowingthrough this element will show an abrupt variation. The current I,flowing through the second rectifier 7 is equal to the differencebetween the currents I and I,.

At the instant at which the current I, flowing through the secondrectifier 7 becomes zero, i.e., at the instant at which the motorcurrent I and the current flowing through the heating element 2 areequal, which instant precedes the instant t, as the supply voltage V-passes through zero, the second rectifier may or may not be triggeredagain. If at this instant t, a new trigger pulse is supplied to thesecond rectifier, this rectifier will remain conducting and the sign ofthe current direction will be reversed. In this case there will continueflowing through the heating element 2 a current the direction of whichis reversed at the instant t At the instant t, at which the motorcurrent I becomes zero, the current I flowing through the heatingelement 2 and the current I, flowing through the second rectifier 7become equal to one another and remain so during the remainder (1 -1 ofthe negative half-cycle of the supply voltage V-. If the secondrectifier is not re-triggered at the instant 1,, the currents I and I,will be zero from this instant. If desired, the second rectifier may beretriggered at an instant later than t,, which permits regulation of theheat dissipation.

A first advantage of the aforedescribed motor speed control permitted bythe arrangement according to the invention is that the power supplied tothe heating element 2 is substantially independent of this speedcontrol. FIG. 4 shows that the waveform of the current I flowing throughthe heating element is substantially sinusoidal. When the instant r, isvaried, the sudden change of this waveform will follow this variation,the overall power supplied to the heating element will be substantiallyconstant. Hence, substantially maximum power is supplied to the heatingelement irrespective of the motor conditions. If the second rectifier 7is not retriggered at the instant t the power supplied to the heatingelement is halved. Thus, triggering of this second rectifier 7 not onlyprovides a motor speed control but also enables the power supplied tothe heating element to be controlled and to be substantially independentof the motor conditions. Triggering the second rectifier 7 alone at thepassages through zero of the supply voltage V- provides maximum heatingin the stationary condition of the motor.

The invention also enables the first rectifier to be triggered at theinstant at which the voltage across it is zero, i.e., at the instant atwhich the supply voltage V- is equal to the back E.M.F. of the motor I.This method of triggering greatly reduces the risk of radio-frequencyinterference.

Also, the variations of the line load will be small because when theheating element is switched to full power, current is supplied to itcontinuously and not, as in the known circuit arrangement, at intervals.

The fact that the first rectifier 3 is a bidirectional rectifier has theadvantage of permitting the direction of rotation of the motor to bereversed by fully electronic means, for triggering this rectifier at theinstant t instead of at the instant t, causes the direction of thecurrent flowing through the motor to be reversed. In this case the speedcontrol is effected by triggering the second rectifier 7 during thenegative half-cycle of the supply voltage.

FIG. 3 shows a second simple embodiment of the arrangement according tothe invention. In this arrangement the first rectifier 3' connected inseries with the motor is a diode. As a result, the motor current isautomatically switched on at the instant at which the voltage acrossthis diode exceeds the threshold value thereof. To enable the directionof rotation of the motor to be reversed a second diode 3 having a passdirection opposite to that of the first diode and a switch 9 areprovided. The direction of current flow through the motor can bereversed by changing over the switch 9 by means of the control device 8.Speed control and regulation of the power supplied to the heatingelement 2 are again effected by means of the second rectifier 7 which istriggered by the control device 8.

FIG. 5 shows, by way of example, an arrangement according to theinvention which includes control circuits. The motor circuit diagram isenclosed in a block M. The trigger pulses for the gates of therectifiers 3 and 7 are obtained by means of a trigger circuit TR and acontrol device D. A square-wave voltage corresponding to a half-cycle ofthe supply voltage V- is derived from the supply voltage V- by means ofthe trigger circuit TR. This square-wave voltage is applied to thecontrol device D, in this case to the base of a transistor Tr Thecollector of this transistor Tr is connected to the base of a transistorTr Two further transistors Tr and Tr, are driven by means of these twotransistors. The collector voltages of the transistors 'Ir, and Tr,determine the voltages at the gates of the rectifiers 3 and 7 and hencetheir conducting or non-conducting conditions. When the positivesquare-wave voltage is applied to the base of the transistor Tr, thecollector of the transistor Tr, will assume a negative potential,permitting the rectifier 3 to become conducting, so that current issupplied to the motor, for example, during the positive half cycle ofthe supply voltage V- When the square-wave voltage collapses thecollector voltage of the transistor Tr, becomes negative and therectifier element 7 becomes conducting, so that current is supplied tothe heating element 2 during the negative halfcycle of the supplyvoltage V-'also.

The speed control is effected by means of a tachogenerator device S.When the first rectifier is conducting, the transistor Tr: isnon-conducting, so that its collector voltage is high. This collectorvoltage is applied to the emitter of the transistor Tr The collectorcurrent of this transistor Tr is determined by the voltage applied toits base by the tachogenerator and is used to charge a capacitor Cconnected between the collector of the transistor Tr, and the base ofthe transistor Tr,. When the voltage across this capacitor has risen toabout 0.7 volts, the transistor Tr becomes conducting and the rectifier7 is triggered. Consequently this triggering instant is determined bythe voltage from the tachogenerator and hence by the motor speed.Obviously, the back E.M.F. of the motor may also be used as anindication of the speed.

A capacitor C is included in the connection between the collector of thetransistor Tr and the base of the transistor Tr-,. The capacitors C andC serve'to limit the durations of the trigger pulses. Should a triggerpulse be applied to the series rectifier 3 during an entire half-cycleof the supply voltage, the motor may be short-circuited, since at theinstant at which the motor current becomes zero this trigger pulse wouldstill be present and at the same time the second rectifier 7 would beconducting, so that the direction of the current flow through the motormay be reversed. This condition persists until the back E.M.F. is zeroand hence the motor is stationary. The capacitors C and C limit thedurations of the trigger pulses, because they are charged via theresistors used. Further capacitors (C and C are provided to suppress theinfluence of interference signals.

To enable the motor to be stopped and a choice to be made betweenmaximum and minimum energy supplied to the heating element the controldevice D includes four transistors Tr Tr Tr and Tr The collector emitterpath of the transistor Tr shunts the base emitter path of the transistorTr When a positive voltage is applied to the base of the transistor Tr,the base emitter path of the transistor Tr is substantiallyshortcircuited, so that the collector voltage of this transistor alwaysis high and the first rectifier 3 is not triggered,

causing the motor to stop. The transistor Tr is connected in series witha base resistor R u of the transistor Tr.,. When this transistor isconducting owing to a positive voltage being applied to its base, thesecond rectifier 7 can be triggered. When the transistor Tr is cut off,the rectifier 7 cannot be triggered, so that power can only be suppliedto the heating element via the motor current.

In the stationary condition of the motor maximum power may be suppliedto the heating element by means of transistors Tr and Tr If, during thewashing program, the motor is in a stationary condition, a positivevoltage is applied to the base of the transistor Tr If maximum heatdissipation is desired, a positive voltage is also applied to a terminalQ and hence to the base of the transistor Tr As a result, during theentire cycle of the supply voltage a trigger pulse is applied to therectifier 7 and hence maximum power is supplied to the heating element.

The cycle of operation of the motor, i.e., the periods during which themotor is required to run and to be stationary, and the reversal of itsdirection of rotation are effected by means of an astable multivibratorA and a bistable multivibrator B. The cycle periods of the astablemultivibrator A may be interchanged by an inverter comprisingtransistors Tr and Tr Depending upon the voltage at a terminal P (baseof the transistor Tr either an end 10 or an end 11 of a resistor R isconnected to the supply via a diode D or a diode D respectively. Thus achoice may be made between a quick operating rhythm of the motor (shortstationary periods and long running period) and a slow operating rhythm(long stationary periods and short running periods).

The output voltage of this astable multivibrator A, which voltagecorresponds to the desired running period, is applied to the bistablemultivibrator B. This ensures reversal of the direction of rotation ofthe motor. The output voltages of this bistable multivibrator areapplied via diodes D and D-, to the trigger circuit and determine duringwhich half-cycle of the supply voltage V- this trigger circuit applies asquare-wave voltage to the control device (base of the transistor Tr Theinput of the astable multivibrator A is also connected to the transistorTr so that during the stationary period of the motor the seriesrectifier 3 is not triggered, and to the transistor Tr so that duringthis stationary period the application or non-application of a positivevoltage to the terminal Q (base of the transistor Tr permits a choice tobe made between maximum and minimum dissipation.

What is claimed is:

l. A control circuit for a self-commutating electric motor comprising, apair of input terminals adapted for connection to a source of supplyvoltage, an electric heating element, a first rectifier, meansconnecting the heating element, the motor and the first rectifier in aseries circuit across said input terminals, a bidirectional controlledrectifier connected across the series combination of the motor and thefirst rectifier and in series with the heating element across the inputterminals,

and first means for selectively applying a first control signal to thecontrol electrode of said bidirectional rectifier to control theconduction period thereof in a manner to regulate the motor current andthereby control the motor speed.

2. A control circuit as claimed in claim 1 wherein the first rectifiercomprises a bidirectional controlled rectifier having a controlelectrode connected to a control device for selectively supplyingcontrol signals to said control electrode to control the conduction ofsaid first rectifier.

3. A control circuit as claimed in claim 1, characterized in that thefirst rectifier comprises a diode.

4. A control circuit as claimed in claim 3 further comprising, a seconddiode connected to the motor with opposite polarity to that of the firstdiode, and a switching element having first and second positions forselectively connecting said first and second diodes, respectively, inseries with the motor and the heating element across said inputterminals.

5. A control circuit as claimed in claim 1 wherein said supply voltageis an AC voltage and said first rectifier comprises a controlledrectifier having a control electrode for initiating conduction therein,second means for applying a second control signal to the controlelectrode of said first rectifier at the time the supply voltage acrossthe first rectifier is approximately zero voltage, said first controlsignal applying means being arranged to supply said first control signalto the control electrode of the bidirectional rectifier during the halfcycle of the supply voltage following said second control signal and atan instant of time which determines the motor speed.

6. A control circuit as claimed in claim wherein said first controlsignal applying means includes means for selectively applying a thirdcontrol signal to the control electrode of the bidirectional rectifierat the start of the next half cycle of the supply voltage.

7. A control circuit as claimed in claim 5 wherein said first controlledrectifier comprises a second bidirectional rectifier and said secondcontrol signal applying means is arranged to selectively apply saidsecond control signal at the positive or negative going zero voltagecrossover of the supply voltage thereby to control the direction ofrotation of the motor.

8. A control circuit as claimed in claim 7 wherein said first controlsignal applying means includes means for selectively applying a thirdcontrol signal to the control electrode of the first bidirectionalrectifier during alternate half cycles of the AC supply voltage thatfollow the half cycles during which the first control signal is appliedto the control electrode of the first bidirectional rectifier.

9. A control circuit as claimed in claim 1 wherein said supply voltageis an AC voltage and said first rectifier comprises a controlledrectifier having a control electrode for initiating conduction therein,and second means for applying a second control signal to the controlelectrode of said first rectifier whereby the motor speed is controlledjointly by said first controlled rectifier and said bidirectionalrectifier.

10. A control circuit as claimed in claim 9 wherein said firstcontrolled rectifier comprises a bidirectional rectifier.

1. A control circuit for a self-commutating electric motor comprising, apair of input terminals adapted for connection to a source of supplyvoltage, an electric heating element, a first rectifier, meansconnecting the heating element, the motor and the first rectifier in aseries circuit across said input terminals, a bidirectional controlledrectifier connected across the series combination of the motor and thefirst rectifier and in series with the heating element across the inputterminals, and first means for selectively applying a first controlsignal to the control electrode of said bidirectional rectifier tocontrol the conduction period thereof in a manner to regulate the motorcurrent and thereby control the motor speed.
 2. A control circuit asclaimed in claim 1 wherein the first rectifier comprises a bidirectionalcontrolled rectifier having a control electrode connected to a controldevice for selectively supplying control signals to said controlelectrode to control the conduction of said first rectifier.
 3. Acontrol circuit as claimed in claim 1, characterized in that the firstrectifier comprises a diode.
 4. A control circuit as claimed in claim 3further comprising, a second diode connected to the motor with oppositepolarity to that of the first diode, and a switching element havingfirst and second positions for selectively connecting said first andsecond diodes, respectively, in series with the motor and the heatingelement across said input terminals.
 5. A control circuit as claimed inclaim 1 wherein said supply voltage is an AC voltage and said firstrectifier comprises a controlled rectifier having a control electrodefor initiating conduction therein, second means for applying a secondcontrol signal to the control electrode of said first rectifier at thetime the supply voltage across the first rectifier is approximately zerovoltage, said first control signal applying means being arranged tosupply said first control signal to the control electrode of thebidirectional rectifier during the half cycle of the supply voltagefollowing said second control signal and at an instant of time whichdetermines the motor speed.
 6. A control circuit as claimed in claim 5wherein said first control signal applying means includes means forselectively applying a third control signal to the control electrode ofthe bidirectional rectifier at the start of the next half cycle of thesupply voltage.
 7. A control circuit as claimed in claim 5 wherein saidfirst controlled rectifier comprises a second bidirectional rectifierand said second control signal applying means is arranged to selectivelyapply said second control signal at the positive or negative going zerovoltage crossover of the supply voltage thereby to control the directionof rotation of the motor.
 8. A control circuit as claimed in claim 7wherein said first control signal applying means includes means forselectively applying a third control signal to the control electrode ofthe first bidirectional rectifier during alternate half cycles of the ACsupply voltage that follow the half cycles during which the firstcontrol signal is applied to the control electrode of the firstbidirectional rectifier.
 9. A control circuit as claimed in claim 1wherein said supply voltage is an AC voltage and said first rectifiercomprises a controlled rectifier having a control electrode forinitiating conduction therein, and second means for applying a secondcontrol signal to the control electrode of said first rectifier wherebythe motor speed is controlled jointly by said first controlled rectifierand said bidirectional rectifier.
 10. A control circuit as claimed inclaim 9 wherein said first controlled rectifier comprises abidirectional rectifier.